The present invention concerns circuits used for communication systems and pertains specifically to a circuit that includes an amplifier and a bypass switch useful for low power devices that utilize microwave signals.
Modem digital wireless systems, such as those used in cellular phones and local area networks (LANs) utilize signals with a frequency typically in the range of 0.5 gigahertz (GHz) to 6.0 GHz.
For strong transmission signals, for example when there is a relatively short transmission distance, it is often advantageous to use a switch or series of switches to bypass a microwave amplifier in order to conserve power. The microwave amplifier, while important to achieve high dynamic range for weak transmission signals, may be unnecessary for strong transmission signals. Providing a bypass of the microwave amplifier allows the system gain to be lowered down when the input signal is strong.
In the prior art, to provide for the bypass of a microwave amplifier, switch components are mounted on a printed circuit board. The switch components route the signal to the amplifier to perform the basic amplification function or to the output. Each switch component is, for example, a mechanical type switch, a solid state field-effect transistor (FET) switch or multiple diodes functioning as a switch. A typical topology of an amplifier with a bypass switch includes three switches. A bypass switch is use to bypass the amplifier. A switch connected to the input of the amplifier and a switch connected to the output of the amplifier are used to isolate the amplifier when the bypass switch is turned on and the amplifier is being bypassed.
One disadvantage of the above-described prior art topology is that every switch and the amplifier requires a separate bias/control circuit. In addition several components must be used to build each switch and amplifier, complicating the system design and using up valuable space. Mechanical switches require high voltage levels (15V or more) and are slow in switching speed. Diode switches require that current be wasted to turn them on and off and require numerous DC bias elements. While switches could be integrated with the amplifier into a single integrated circuit (IC), the control and signal degradation due to parasitic capacitances within such an IC could degrade the performance of the IC.
Ray Moroney, Kevin Harrington, Wayne Struble, Brian Khabbaz, Mike Murphy, A High Performance Switched-LNA IC for CDMA Handset Receiver Applications, 1998 IEEE Radio Frequency Integrated Circuits Symposium, p. 43-46, shows an integrated Gallium Arsenide (GaAs) FET solution where a switch function is integrated with an amplifier. However, the disclosed circuit uses an external control for the switch function. Additionally, the disclosed amplifier/switch uses only a series mode switch. Also, the disclosed circuit uses several blocking capacitors that increase die size and cost.